This application is a national stage of PCT/DE99/03683 filed Nov. 18, 1999 and is based upon German national application 198 53 023.4 of Nov. 18, 1998 under the International Convention.
1. Field of the Invention
The invention relates to a method of producing a layer having a submicrometer structure. The invention further relates to a component such as a sensor, switch or optical device.
2. Background of the Invention
It is known, as state of the art, to produce submicrometer structures by the use of optical lithography. In this, the desired nanostructuring is formed with the aid of exposure-supported mask technology. The use of long wave UV light enables a resolution of up to 180 nm in projection exposure with expensive steppers.
Known methods with better resolution, like x-ray lithography, electron-beam projection lithography, ion beam lithography or the imprint process are in development (L. R. Harriott, Materials Science in Semiconductor Processing 1 (1998) 93-97). These processes have the disadvantage that they involve high technical expenditures and associated high cost.
It is thus desirable to develop methods which are based upon self-organization or other self-controlling processes. To produce a high output of such structures in production, it is desirable to utilize methods which can be carried out in parallel. In the patent application PCT/DE 96/00172, a method of structuring silicide films is described which is based upon local oxidation of silicides. The disadvantage is that the method has only practical utility for monocrystalline silicide layers. In polycrystalline silicide layers during the requisite oxidation at high temperatures, disadvantageous morphological holes are formed.
It is thus the object of the invention to provide a method for producing a nanostructure which allows nanostructures to be produced also in polycrystalline layers with a self-controlling process, i.e. without the need to write the small structures directly therein.
According to the invention, for producing a layer having a submicrometer structure on a substrate, initially a layer is formed on a substrate. It is known to then form, by appropriate means elastic stresses at at least one of the predetermined positions for structuring of this layer. Within the framework of the invention it has been recognized that the layer can be subjected to a stress-dependent solid-state reaction.
The elastic stresses are so formed that as a result in the layer in the subsequent solid-state reaction, by diffusion processes there will be a stress-oriented enrichment of the layer material at this position. As a result, the layer is interrupted, thereby imparting a structure to the layer. Depending upon the selection of the parameters, for example, the layer thickness, the layer material and the solid-state reaction parameters like temperature and atmosphere, the type of structuring can be established.
It can be sufficient that the solid-state reaction is limited to a few monolayers analogous to that described in the publication xe2x80x9cKaren Maex, Materials Science and Engineering RII (1993) 53-153xe2x80x9d, examples according to FIG. 49 (Ti on SiO2, Co on SiO2).
For forming the elastic stresses, a mask can be formed on the layer to be structured with one or more mask edges. The stresses according to the invention are formed in the layer as a precondition for the structuring by the solid-state reaction. It is however also possible, alternatively or cumulatively to locally so modify the substrate material of the substrate carrying the layer that the stresses are formed at the corresponding layer position. Such a modification can be effective in the form of an ion implantation or by a thermal treatment (layer treatment) either before or after the formation of the layer on the substrate.
It has been found within the framework of the invention that conventional optical lithography can be used for structuring a mask formed on a film. It has been found that along the linearly running mask edges of such a mask stress lines or stress points are formed on the one hand on the film to be structured and on the other hand during application of the solid-state reaction, that this reaction is effected in a stress-dependent manner in the thin film.
It has also been found that the diffusion process during the solid-state reaction, for example for alloy formation, especially silicide formation, is so modified by an elastic stress profile of the mask that a small line (for example 50 nm) is formed along the mask edge in a self-adjusting manner.
The here proposed method serves for the production of the smallest critical structures in the submicrometer range. The method of the invention can be used for the formation of microelectronic or optoelectronic circuitry. The larger structures (typically greater than 0.2 micrometers) can be formed by means of known photolithographic processes. The method of the invention is suitable for generating the nanostructures directly and self-adjusting to the mask edges.
The method of the invention is used especially advantageously for the silicide-on-silicon material system. For example, with the aid of the method of the invention a cobalt disilicide (CoSi2) layer can be deposited on a silicon substrate and structured. Based upon surprising conductivity, the high thermal stability and compatibility with silicon, CoSi2 in addition to TiSi2 has been found to be an especially important material for contacting of microchips. Source, drain and gate contacts can be fabricated with these polycrystalline silicides. For the constantly smaller structural sizes CoSi2 has significant advantages over TiSi2 since the resistance of the conductive paths is independent on the line width (at least to 100 nanometers). Silicides can also be used for structured layer formation also for other microelectronic or solid-state electronic purposes. For example, the use in Schottky barrier photodetectors is advantageous. As a consequence, structures with minimal dimensions, especially in the range of 50 nm or even smaller can be achieved with a method for the structuring layers for nanotechnology in accordance with the method of the invention. The method serves advantageously for the conventional optical lithography in the structuring of a thin film of a mask formed from a thin film (especially a nitride layer) and a stress-dependent solid-state reaction in the ultrathin film formed on a substrate. The solid-state reaction (for example silicide formation) is so modified by the elastic stress profile of the mask (nitride layer) that a small uniform line is formed in thin film (for example silicide) along mask edges in a self-adjusting manner. The line width is adjustable in the sense of the invention. The adjustability of the dimensions of the structure is a function of the film thickness, the thermal treatment and the mask construction. It is possible to form structures with dimensions in the range of 50 to 100 nm.
The advantages of the method of the invention lie in the high resolution capacity of the structuring. The method also enables a high output since the process is not dependent on the size of the substrate carrying the film to be structured. The method involves technologically simple processes and has good reproducibility. The special advantage is that it is not necessary to use submicrometer masks which are expensive to fabricate, for forming submicrometer structures, especially nanostructures. For the use of the method in silicon technology, it suffices to use standard silicon semiconductor standards without requiring the expensive further procedures. Finally, the method of the invention can also be translated to other material systems and other stress-dependent solid-state reactions. The invention can find application in the field of nanoelectronics for the formation of switching elements like, for example, the formation of nanoMOSFETs. The invention can also be used for the production of sensors having nanostructures especially in the field of optical components. It is however also possible to make masks for nanostructures by the process.
The invention is described in greater detail in conjunction with FIG. 1 and specific examples.